Our objective is to investigate the underlying mechanism in the development and differentiation of malaria blood-stage parasites. The blood-stage parasites propagate through asexual division (merozoites) and sexual differentiation, which upon intake by mosquitoes will fertilize and preserve its complex life cycle. To initiate the study we have established an in vitro culture of human malaria parasites, Plasmodium falciparum, in our laboratory. Parasites were cultivated in human erythrocytes and harvested at different stages. Monoclonal antibody to a major sexual stage antigen, pfs25, has been identified. Development of other monoclonal antibodies against stage-specific antigens are ongoing. The antibodies will be used in the study of stage differentiation. Besides Plasmodium stage-specific antigens, we have also looked into the function of heat shock proteins (hsps) in parasite development. Hsps are a family of proteins expressed either under stress conditions or constitutively and are well defined among higher eukaryotes. Recent studies of infectious diseases have suggested hsps may play an important role in pathogen development and survival in host. We have previously isolated a hsp90 gene and are trying to elucidate its structure. Other hsps of malaria parasite include major hsp70 and chaperonin hsp60. Their expressions in individual stages are currently under investigation.